// Copyright (c) 2012 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.

#include "leveldb/filter_policy.h"

#include "leveldb/slice.h"
#include "util/hash.h"

namespace leveldb
{

    namespace
    {
        static uint32_t BloomHash(const Slice &key)
        {
            return Hash(key.data(), key.size(), 0xbc9f1d34);
        }

        class BloomFilterPolicy : public FilterPolicy
        {
        public:
            explicit BloomFilterPolicy(int bits_per_key) : bits_per_key_(bits_per_key)
            {
                // We intentionally round down to reduce probing cost a little bit
                k_ = static_cast<size_t>(bits_per_key * 0.69); // 0.69 =~ ln(2)
                if (k_ < 1)
                    k_ = 1;
                if (k_ > 30)
                    k_ = 30;
            }

            const char *Name() const override { return "leveldb.BuiltinBloomFilter2"; }

            void CreateFilter(const Slice *keys, int n, std::string *dst) const override
            {
                // Compute bloom filter size (in both bits and bytes)
                size_t bits = n * bits_per_key_;

                // For small n, we can see a very high false positive rate.  Fix it
                // by enforcing a minimum bloom filter length.
                if (bits < 64)
                    bits = 64;

                size_t bytes = (bits + 7) / 8;
                bits = bytes * 8;

                const size_t init_size = dst->size();
                dst->resize(init_size + bytes, 0);
                dst->push_back(static_cast<char>(k_)); // Remember # of probes in filter
                char *array = &(*dst)[init_size];
                for (int i = 0; i < n; i++)
                {
                    // Use double-hashing to generate a sequence of hash values.
                    // See analysis in [Kirsch,Mitzenmacher 2006].
                    uint32_t h = BloomHash(keys[i]);
                    const uint32_t delta = (h >> 17) | (h << 15); // Rotate right 17 bits
                    for (size_t j = 0; j < k_; j++)
                    {
                        const uint32_t bitpos = h % bits;
                        array[bitpos / 8] |= (1 << (bitpos % 8));
                        h += delta;
                    }
                }
            }

            bool KeyMayMatch(const Slice &key, const Slice &bloom_filter) const override
            {
                const size_t len = bloom_filter.size();
                if (len < 2)
                    return false;

                const char *array = bloom_filter.data();
                const size_t bits = (len - 1) * 8;

                // Use the encoded k so that we can read filters generated by
                // bloom filters created using different parameters.
                const size_t k = array[len - 1];
                if (k > 30)
                {
                    // Reserved for potentially new encodings for short bloom filters.
                    // Consider it a match.
                    return true;
                }

                uint32_t h = BloomHash(key);
                const uint32_t delta = (h >> 17) | (h << 15); // Rotate right 17 bits
                for (size_t j = 0; j < k; j++)
                {
                    const uint32_t bitpos = h % bits;
                    if ((array[bitpos / 8] & (1 << (bitpos % 8))) == 0)
                        return false;
                    h += delta;
                }
                return true;
            }

        private:
            size_t bits_per_key_;
            size_t k_;
        };
    } // namespace

    const FilterPolicy *NewBloomFilterPolicy(int bits_per_key)
    {
        return new BloomFilterPolicy(bits_per_key);
    }

} // namespace leveldb
